The invention relates to a catalytic composition and a process for the selective methanation of carbon monoxide, in particular for use in fuel cell systems.
Low-temperature fuel cells can only be operated using hydrogen or hydrogen-rich gases of defined quality. The CO concentration depends on the energy carrier employed and on the reforming process used. The removal of relatively high CO concentrations can be effected by means of the shift process with further formation of hydrogen, However, a residual CO concentration, generally in the range from 0.5 to 1.5% by volume, remains, depending on the process design. When Cu catalysts are used, CO removal down to 3000 ppm can, for example, be made possible. The CO content of the hydrogen-rich gas has to be reduced further as far as possible in order to avoid poisoning of the anode catalyst.
The removal of the comprised CO from the gas stream down to below the required limit value is usually carried out in a fine purification step Selective oxidation is nowadays the customary CO removal method. The selective oxidation is highly developed but has the disadvantages of not only moderate selectivity but also the necessity of precisely metered introduction of air, resulting in a high outlay for instrumentation. In addition, mixing the oxidant oxygen into the gas is problematical in terms of safety. The removal of the CO by reaction with H2 (methanation) has considerable advantages over the selective oxidation of CO because it can be realized without any great demands in terms of process engineering.
The methanation of CO (hydrogenation of carbon monoxide to methane) proceeds according to the reaction equation:CO+3H2→CH4+H2O ΔH=−206.2 kJ/mol
A competing reaction which occurs is the conversion of CO2 into methane:CO2+4H2→CH4+2H2O ΔH=−164.9 kJ/mol
The particular challenge for the selective methanation of CO is that CO should be hydrogenated preferentially and CO2 should not be hydrogenated, since this would consume further hydrogen. Thermodynamically, the methanation of CO is preferred over the methanation of CO2. It is known that methanation of CO2 does not occur below a limit value of 200-300 ppm of CO in the combustion gas. The CO concentration in the combustion gas is about 10 000 ppm, i.e. a factor of 50 higher than the limit indicated.
The CO2 content is from about 15 to 25% by volume and thus an order of magnitude above the CO content. Accordingly, a CO-selective catalyst is indispensable.
The selective methanation of CO has been known for a long time. CO was firstly methanated over an Ni catalyst, but CO2 had to be scrubbed out beforehand. In 1968, a ruthenium catalyst for the selective methanation of CO was claimed by Baker et al. (U.S. Pat. No. 3,615,164) who used a ruthenium or rhodium catalyst on an aluminum oxide support material. Likewise, the selective methanation of CO in a gas mixture comprising hydrogen, carbon dioxide and carbon monoxide at temperatures in the range from 125 to 300° C. using ruthenium-comprising catalysts is described in Chemical Abstracts, Volume 74, 1971, No. 35106u. U.S. Pat. No. 3,663,162 of 1972 claims a Raney nickel catalyst for this reaction.
In EP-A-1174486, a methanation stage is combined with a unit for selective oxidation with the objective of a lower oxygen consumption and a lower degree of methanation of CO2.
In EP-A-0946406, two methanation stages having different temperature levels are connected to one another. An advantage here is said to be that no or little CO2 is methanated in the high-temperature stage but a large part of the carbon monoxide is reacted in this stage. The removal of the remaining CO occurs in the subsequent low-temperature methanation.
WO 97/43207 describes the combination of a first stage for selective oxidation with a subsequent methanation stage. This combination is said to allow both processes to be operated under optimal conditions.
Further more recent patent applications, for example EP-A-1246286, in which a methanation reactor is preferred over a selective oxidation unit as last process stage of a gas purification for reasons of simpler construction and simpler operability, likewise describe optimized process stages but use conventional catalysts, predominantly catalysts based on ruthenium or nickel.
JP-A-2004097859 describes catalysts for the removal of CO in hydrogen-comprising gas streams by reaction with H2. As catalysts, mention is made of inorganic supports to which one or more metals selected from the group consisting of Ru, Ni and Co have been applied. Support materials are TiO2, ZrO2, Al2O3 and zeolites.
JP-A-2002068707 relates to a process for removing CO from hydrogen-comprising gas by selective methanation of the CO using a catalyst comprising an Ru component and an alkali metal and/or alkaline earth metal on a heat-resistant inorganic oxide support.
The use of carbon as catalyst support has hitherto not been described for the methanation of carbon monoxide.
The processes of the prior art do not allow a sufficient reduction in the CO content to be obtained while preserving the CO2 content. The catalysts proposed are either not selective enough or work only within a narrow temperature range.
The exothermic nature of the reaction results in hot spots. For this reason, it has to be possible to operate within a wide temperature window. Another problem is the adiabatic temperature increase in monoliths when these are used as shaped catalyst bodies, which is the case in industrial practice.
For fuel cell applications in particular, the required maximum CO content in the hydrogen-rich gas fed in and the necessary high selectivity (methanation of CO but not of CO2) over a wide temperature window still provide a great potential for development of suitable deactivation-resistant catalysts.